The method is safe, and women already get frequent ultrasound scans during pregnancy, said study co-author Dr. Vincenzo Berghella, director of maternal fetal medicine at Thomas Jefferson University Hospital in Philadelphia.

"This is something that is easily implementable tomorrow if people wanted," Berghella told Live Science.

Guessing game, waiting game

Though most women are given an official due date, it is often just a rough estimate. Only 5 percent of women deliver on their due date, according to American Academy of Pediatrics.

A healthy pregnancy is, on average, 40 weeks from the last menstrual period, but in reality, most babies come sometime between 37 and 42 weeks gestation.

For many women, that five-week window creates a waiting game that can feel unbearable. Women with little paid maternity leave may want to work until the last possible moment they can. Those whose due date has come and gone may wonder whether they should have labor induced with drugs. And pregnant women who already have other children often scramble to find child care during labor.

But there haven't been reliable methods to tell a woman exactly when her baby is coming. Cervical checks, in which a doctor manually feels the length and softness of the cervix with his or her fingers, are typically not very reliable, Berghella said.

However, studies have shown that women who have a short cervix, less than 1 inch (2.5 centimeters) long, as measured on ultrasound scans during mid-pregnancy, are at high risk for preterm birth. That made researchers wonder whether similar scans could be predictive during the homestretch of pregnancy.

Measurable changes

To answer that question, Berghella looked at five studies in which researchers used ultrasounds to measure cervical length of a total of 735 pregnant women. He found that among the women whose cervix was more than 1.18 inches (3 cm) long at the time it was measured, less than 50 percent of these women delivered within a week.

By contrast, among the women whose cervixes were less than 0.4 inches (1 cm) long, 85 percent delivered within a week, the researchers reported today (Oct. 27) in the BJOG: An International Journal of Obstetrics and Gynecology.

"The beginning of changes in the cervix happen internally, not externally," Berghella told Live Science.

That's why manual exams don't predict labor very well: Because the changes are happening internally, doctors can't feel them from the outside. However, transvaginal ultrasounds reveal the shape of the entire cervix, allowing doctors to see if the process has started, Berghella said.

Better medical planning

Estimating due dates more closely could help doctors make medical decisions. For babies with congenital anomalies who must be delivered in a hospital, getting a better estimate of the due date could ensure safer delivery. And hospitals could make arrangements to have more staff on hand if they know lots of deliveries are imminent in the next week, Berghella said.

Sometimes, women have a medical condition that necessitates delivering their baby soon, but not necessarily immediately. In those instances, doctors may take a wait-and-see approach to see if a woman is going to deliver soon, but may opt to induce if they feel labor is not imminent, he said.

"If her cervix is short, you could say, 'Oh, you can probably wait; you are probably going to go into labor,'" Berghella told Live Science.

Tia has interned at Science News, Wired.com, and the Milwaukee Journal Sentinel and has written for the Center for Investigative Reporting, Scientific American, and ScienceNow. She has a master's degree in bioengineering from the University of Washington and a graduate certificate in science writing from the University of California Santa Cruz.